Hi all, thanks for your advice. It turned out to be really simple. Rather than using the system variable, Woocommerce allows you to specify the logo you’d like on the WooCommerce / Settings / Emails page. I guess it’s a feature but I’d not expected it. Also, it doesn’t allow a relative path (that is, /wp-content/…) but requires the full domain path. I added http://www.wavez.com.au/ to the beginning of it and it’s working now.
Sorry, in my cutting-pasting it seems that my PC changed the x in 225×54.png to 225×54.png.
This link works:
https://wavez.com.au/wp-content/uploads/2019/05/WAVEZ-DARK-LOGO-TRANS-225×54.png